Display apparatus and echo cancellation method thereof

ABSTRACT

A display apparatus is provided. The display apparatus includes a display, a video receiver, and a processor. The display is configured to display an image. The video receiver is configured to receive an input signal of an impulse response. The processor configured to divide the input signal into sub-bands, apply primary echo-cancellation to the input signal by using sub-band signals corresponding to the sub-bands, estimate a residual echo based on acoustic echo path information obtained in the primary echo-cancellation with regard to each of the sub-bands, and perform secondary echo-cancellation to remove the estimated residual echo from the primary-echo-canceled input signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Korean Patent Application No.10-2014-0185742, filed on Dec. 22, 2014 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference.

BACKGROUND

1. Field

Apparatuses and methods consistent with exemplary embodiments relate toa display apparatus and an echo cancellation method thereof, and moreparticularly to a display apparatus which can suppress a residual echoand an echo cancellation method thereof.

2. Description of the Related Art

A display apparatus such as a television (TV), a portable apparatus suchas a mobile terminal, or a similar electronic apparatus can supportfunctions such as voice recognition for recognizing a user's voice tocontrol a device, a video call or video conference using a user's voice,etc. and a use range of these functions based on a voice have beengradually extended. The apparatus supporting such functions uses amicrophone to receive a voice signal, and the received voice signal mayinclude an echo.

The echo is results from a sound wave propagating from a sound sourceand reflecting off an object. Such an echo is easy to hear in dailylife, and there is a sound reverberating from a mountain as an exampleof a simple echo based on a single reflection.

In contrast with the echo, there is a direct sound. The direct soundrefers to a sound that is heard directly without being reflected off anobject. Hence, the echo is a reflection of sound arriving apredetermined time after the direct sound.

In general, a sound made inside a structure with reflective surfaces,such as a room where a TV is installed, is reflected many times andbecomes complex since directions of reflected sounds are all different.This is an example of multiple echoes, and is one of causes of aresidual echo. For instance, the residual echo may be caused when asound output from a speaker is reflected many times and then returned toa microphone.

If an echo and a residual echo are not properly canceled from an inputvoice signal, it may inconvenience a user who is using a correspondingfunction and may cause an error in operation. For a normal operation,not only the echo but also the residual echo has to be canceled from thevoice signal.

However, the existing cancellation for the echo or the residual echobecomes a burden to an apparatus in light of a processing time or loadsince the existing cancellation generally needs complicatedcalculations, and has a disadvantage that the residual echo is noteffectively canceled.

SUMMARY

According to an aspect of an exemplary embodiment, there is provided adisplay apparatus including a display configured to display an image; avideo receiver configured to receive an input signal of an impulseresponse; and at least one processor configured to divide the inputsignal into a plurality of sub-bands, apply primary echo-cancellation tothe input signal by using a plurality of sub-band signals correspondingto the plurality of sub-bands, estimate a residual echo based onacoustic echo path information obtained in the primary echo-cancellationwith regard to each of the plurality of sub-bands, and perform secondaryecho-cancellation to remove the estimated residual echo from theprimary-echo-canceled input signal.

The at least one processor may include an acoustic echo canceler thatperforms the primary echo-cancellation to obtain the acoustic echo pathinformation, and a residual echo suppressor that performs the secondaryecho-cancellation based on the acoustic echo path information obtainedby the acoustic echo canceler.

The acoustic echo canceller may estimate the acoustic echo pathinformation for each of the sub-bands, and the residual echo suppressormay determine a decaying coefficient of an impulse responsecorresponding to each of the plurality of sub-bands, which hasexperienced the primary echo-cancellation, based on the estimatedacoustic echo path information for the sub-bands.

The residual echo suppressor may determine a maximum of the impulseresponse and a local maximum at a decaying tail of the impulse responsefor each sub-band, and determine the decaying coefficient based on thedetermined maximum and local maximum.

The residual echo suppressor may determine the maximum and local maximumfor each sub-band by dividing the impulse response corresponding to eachsub-band into a plurality of search windows and updating the maximum orthe local maximum while increasing an index with regard to each dividedsearch window.

The residual echo suppressor may skip a procedure for determining thelocal maximum in a corresponding search window if the maximum isdetermined in a beginning of the impulse response, and update the localmaximum in a next search window.

The decaying coefficient may be determined byρ_(b)=f_(sb)·ln(H_(max)/Max₁)/(i₁−i_(max)), where, H_(max) denotes amaximum of the impulse response, i_(max) denotes an index of H_(max),Max₁ denotes an ultimately updated local maximum, i₁ denotes an index ofthe local maximum, and f_(sb) denotes a sampling frequency due todown-sampling of the impulse response.

The residual echo suppressor may estimate initial power in a finalresidual echo section of the impulse response corresponding to each ofthe plurality of sub-bands, which has experienced the primaryecho-cancellation, based on the decaying coefficient.

The initial power in each of the plurality of sub-bands may bedetermined by

$c_{b} = {\exp \left\{ {- \frac{2\rho_{b}N_{w}}{f_{sb}}} \right\} {\sum\limits_{j = 0}^{N_{w} - 1}\; {{{\hat{h}}_{b}^{2}\left( {L_{b} - N_{w} + j} \right)}.}}}$

where, Cb denotes initial power, ρ_(b) denotes a decaying coefficient ofa sub-band impulse response, L_(b) denotes a length of the sub-bandimpulse response, N_(w) denotes a length of a decaying tail of thesub-band impulse response, f_(sb) denotes a sampling frequency due to adown-sampling of an impulse response, and ĥ_(b) (n) denotes the sub-bandimpulse response.

The residual echo suppressor may estimate an acoustic echo path byapplying adaptive filtration to each of the plurality of sub-bands,estimate an echo by convolving the estimated acoustic echo path for thesub-bands with a speaker signal, and remove a full-band echo signalsynthesized with an echo estimated for the sub-bands from the inputsignal.

According to an aspect of another exemplary embodiment, there isprovided an echo cancellation method of a display apparatus, the methodincluding receiving an input signal of an impulse response; dividing theinput signal into a plurality of sub-bands, and applying primaryecho-cancellation to the input signal by using a plurality of sub-bandsignals corresponding to the plurality of sub-bands; and estimating aresidual echo based on acoustic echo path information obtained in theprimary echo-cancellation with regard to each of the plurality ofsub-bands, and performing secondary echo-cancellation to remove theestimated residual echo from the primary-echo-canceled input signal.

The applying the primary echo-cancellation to the input signal mayinclude estimating acoustic echo path information for each of theplurality of sub-bands, and the performing the secondaryecho-cancellation may include determining a decaying coefficient of animpulse response corresponding to each of the plurality of sub-bands,which has experienced the primary echo-cancellation, based on theestimated acoustic echo path information for the sub-bands.

The performing the secondary echo-cancellation may include determining amaximum of the impulse response and a local maximum at a decaying tailof the impulse response for each sub-band, and determining the decayingcoefficient based on the determined maximum and local maximum.

The maximum and local maximum may be determined for each sub-band bydividing the impulse response corresponding to each sub-band into aplurality of search windows and updating the maximum or the localmaximum while increasing an index with regard to each divided searchwindow.

The performing the secondary echo-cancellation may include skipping aprocedure for determining the local maximum in a corresponding searchwindow if the maximum is determined in a beginning of the impulseresponse; and updating the local maximum in a next search window.

The decaying coefficient may be determined byρ_(b)=f_(sb)·ln(H_(max)/Max₁)/(i₁−i_(max)), where, H_(max) denotes amaximum of the impulse response, i_(max) denotes an index of H_(max),Max₁ denotes an ultimately updated local maximum, i₁ denotes an index ofthe local maximum, and f_(sb) denotes a sampling frequency due todown-sampling of the impulse response.

The performing the secondary echo-cancellation may include estimatinginitial power in a final residual echo section of the impulse responsecorresponding to each of the plurality of sub-bands, which hasexperienced the primary echo-cancellation, based on the decayingcoefficient.

The initial power in each of the plurality of sub-bands may bedetermined by

$c_{b} = {\exp \left\{ {- \frac{2\rho_{b}N_{w}}{f_{sb}}} \right\} {\sum\limits_{j = 0}^{N_{w} - 1}\; {{{\hat{h}}_{b}^{2}\left( {L_{b} - N_{w} + j} \right)}.}}}$

where, Cb denotes initial power, ρ_(b) denotes a decaying coefficient ofa sub-band impulse response, L_(b) denotes a length of the sub-bandimpulse response, N_(w) denotes a length of a decaying tail of thesub-band impulse response, f_(sb) denotes a sampling frequency due to adown-sampling of the impulse response, and ĥ_(b) (n) denotes thesub-band impulse response.

The applying the primary echo-cancellation may include estimating anacoustic echo path by applying adaptive filtration to each of theplurality of sub-bands; estimating an echo by convolving the estimatedacoustic echo path for the sub-bands with a speaker signal; and removinga full-band echo signal synthesized with an echo estimated for thesub-bands from the input signal.

According to an aspect of another exemplary embodiment, there isprovided an echo cancellation device including at least one processorconfigured to perform primary echo-cancellation on an input signal bydividing the input signal into a plurality of side-bands, removing aprimary echo from each of the plurality of side-bands to produce aprimary-echo-cancelled signal for each side-band, and estimating aresidual echo for each of the primary-echo-cancelled signals; and atleast one processor configured to perform residual echo-cancellation bygenerating a full-band residual echo signal from the estimated residualechoes for each side-band, and remove the full-band residual echo signalfrom a signal on which the primary echo-cancellation has been performed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and/or other aspects will become apparent and more readilyappreciated from the following description of exemplary embodiments,taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of an echo cancelation device provided in anelectronic apparatus according to an exemplary embodiment;

FIG. 2 is a block diagram schematically showing an echo cancelationdevice according to an exemplary embodiment;

FIG. 3 is a block diagram showing an echo cancelation device thatcomplements operations of a residual echo suppressor of the echocancellation device of FIG. 2 according to an exemplary embodiment;

FIG. 4 is a block diagram showing an echo cancelation device thatcomplements operations of the residual echo suppressors of the echocancellation devices of FIG. 2 and FIG. 3 according to an exemplaryembodiment;

FIG. 5 is a view showing a waveform of an impulse response signalaccording to an exemplary embodiment; and

FIG. 6 is a flowchart showing operations based on an algorithm forestimating a sub-band impulse response decaying coefficient according toan exemplary embodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, exemplary embodiments will be described in detail withreference to accompanying drawings so as to be easily materialized by aperson having an ordinary skill in the art. The inventive concept may beembodied in various forms and is not limited to the following exemplaryembodiments. For convenience of description, parts not directly relatedto the present disclosure are omitted, and like numerals refer to likeelements throughout.

FIG. 1 is a block diagram of an echo cancelation device provided in anelectronic apparatus according to an exemplary embodiment;

As shown in FIG. 1, an electronic apparatus 10 according to an exemplaryembodiment may include a voice receiver 11 for receiving a voice signalof a voice of a person S, a sound output 12 for outputting a voicesignal, and an echo cancelation device 100 for canceling an echo fromthe received voice signal. While a voice signal of the voice of theperson is described, it will be understood that a sound signal of asound other than a voice may be received and an echo cancelled from thereceived sound signal.

The voice receiver 11 may include a microphone to receive an inputsignal, and the sound output 12 may include a speaker to output anoutput signal.

The echo cancelation device 100 includes at least one processor. Asshown in FIG. 1, the echo cancelation device 100 may for example includean acoustic echo canceler (AEC) 110 as a processor for first echocancellation (as a first processor), and a residual echo suppressor(RES) 120 as another processor for second echo cancellation (as a secondprocessor).

The at least one processor may load a relevant program from anonvolatile memory (e.g., a read only memory, ROM), where programs arestored, to a volatile memory (e.g., a random access memory, RAM) andexecute the loaded program.

The at least one processor may be implemented by combination between aprogram for implementing at least one algorithm (to be described later)and a chip, for example, an integrated chip (IC) provided as a dedicatedprocessor for executing the program.

For example, according to an exemplary embodiment, the acoustic echocanceler 110 and the residual echo suppressor 120 may be implemented bytwo chips that operate based on their corresponding algorithms,respectively, or both the acoustic echo canceler 110 and the residualecho suppressor 120 may be implemented by a single chip. In the casewhere both the acoustic echo canceler 110 and the residual echosuppressor 120 are provided as the single chip, one processor mayoperate to run a program for the first echo cancellation and a programfor the second echo cancellation.

According to an exemplary embodiment, the echo cancelation device 100may be include a central processing unit (CPU), an application processor(AP), a microcomputer (MICOM), or the like. Alternatively, the echocancelation device 100 may be implemented by a general-purpose processorso that the single processor may, for example, load a programcorresponding to an algorithm stored in the ROM to the RAM and executethe loaded program in order to, for example, independently perform thefirst echo cancellation and the second echo cancellation. That is, oneprocessor operates to run both the program for the first echocancellation and the program for the second echo cancellation.

For instance, if the echo cancelation device 100 is implemented by theCPU, the CPU may perform not only the acoustic echo cancellation butalso various functions supported in the electronic apparatus 10, forexample, control over various image processing processes such asdecoding, demodulating, scaling, etc. with regard to an image displayedon a display; response to a command received from a user input includinga remote controller; control over wired/wireless network communication;etc. The functions supported by the processor may include controloperations corresponding to voice recognition for recognizing a voicesignal from which an echo is canceled according to an exemplaryembodiment (for example, channel change, volume control, etc. in a TV),or operations for a video call (for example, making a caller image inputthrough a video receiver, e.g. a camera and an image of the other partyreceived through a communicator be displayed on a screen; transmitting acaller' voice signal input through the voice receiver 11, from which anecho is canceled, to the other party; and outputting a voice signalreceived from the other party to the sound output 12.

In this exemplary embodiment, the electronic apparatus 10 is a displayapparatus such as a TV or a set-top box. The display apparatus uses animage processing process to process an image signal provided from atleast one external image source such as a broadcasting station to bedisplayed as an image on the display. The image processing process maybe preset.

This exemplary embodiment relates to a display apparatus such as a TVfor displaying a broadcast image based on a broadcast signal/broadcastinformation/broadcast data received from a transmitter of a broadcastingstation. However, the kind of images displayable by the displayapparatus is not limited to the broadcast image. Alternatively, thedisplay apparatus may for example display a moving image, a still image,an application, an on-screen display (OSD), a user interface (UI,hereinafter also referred to as a graphic user interface (GUI)), etc.based on a signal/data received from various image sources.

According to an exemplary embodiment, the display apparatus may be asmart TV or an Internet protocol (IP) TV. The smart TV is capable ofreceiving and displaying a broadcast signal in real time, and has aweb-browsing function for searching and consuming various contentsthrough Internet while displaying the broadcast signal in real time. Tothis end, the smart TV offers a convenient environment to a user.Further, the smart TV has an open software platform and thus providesinteractive services to a user. Therefore, the smart TV can offervarious contents, for example, an application of providing apredetermined service, to a user through the open software platform.Such an application is an application program capable of providingvarious kinds of service, which may for instance include applicationsfor providing social network services (SNS), finance, news, weather,maps, music, movies, games, electronic books, etc.

The display apparatus according to an exemplary embodiment may providean application for performing voice recognition and/or a video call. Forthe voice recognition, a voice recognition engine for applying the voicerecognition to an input voice signal may be provided inside the displayapparatus or outside the display apparatus (for example, in a cloudserver).

The present inventive concept may be applied to another displayapparatus, for example, a monitor connected to a computer, etc.

In addition, the electronic apparatus 10 according to an exemplaryembodiment is not limited to a TV, i.e. a display apparatus.Specifically, the electronic apparatus 10 may be a portable apparatussuch as a mobile terminal including a smart phone or the like cellularphone, a tablet personal computer (smart pad), a portable media player(MP3 player), a digital camera, a camcorder, etc. but is not limitedthereto.

For a user's convenience, the portable apparatus such as the cellularphone may support a hands-free call using the microphone and the speakerprovided separately from those in a handset. Such a hands-free functionis applicable to the portable apparatus by various manners such as ahands-free phone for a vehicle, a teleconference system, a speakerphonesystem, etc. Further, the portable apparatus may also support the voicerecognition and/or the video call and use an internal or external voicerecognition engine for the voice recognition.

Accordingly, the echo cancelation device 100 provided in the electronicapparatus 10 according to an exemplary embodiment may be employed forcanceling an echo from an input signal while performing the foregoinghands-free function as well as voice recognition and/or a video call.

FIG. 2 is a block diagram schematically showing an echo cancelationdevice according to an exemplary embodiment.

As shown in FIG. 2, in a telecommunication system using an electronicapparatus 200, a far-end speaker (i.e. far-end user) can hear his/herown voice (i.e. a reflected sound) delayed due to an acoustic pathbetween a sound producing device and microphone 21 at a near-end. Thisreflected sound is referred to as an acoustic echo. To control theacoustic echo, acoustic echo cancellation is used.

Usually the acoustic echo is analyzed at the near-end, where a speakerand a microphone share the same acoustic environment. Echo cancellationin a real case is complicated by additional noise sources, for example,near-end speech s(t) or sounds v(t) produced by working devices. Allthese sounds are summed into a microphone signald(t)=h(t)*x(t)+s(t)+v(t), where h(t)*x(t) denotes the acoustic echo,produced as the convolution of an acoustic echo path h(t) and a speakersignal x(t). The speaker signal x(t) is also referred to as a far-endspeaker signal or a reference signal, and the microphone signal, i.e.,the convolution of an input signal and an echo signal, is given in theform of an impulse response (IS).

Therefore, an acoustic echo canceller (AEC) 210 according to anexemplary embodiment operates to perform echo suppression on the inputsignal d(t) received in the microphone, while preserving at least thenear-end speech s(t) corresponding to a user's direct sounds.

The operations of the acoustic echo canceler (AEC) 210 are based on afinite impulse response (FIR) adaptive filter (AF), which is adapting bya certain algorithm to the echo path h(t). For example, the acousticecho canceler (AEC) 210 may operate to adjust a filter coefficient ofthe adaptive filter to thereby decrease an error between a real echocomponent and an echo signal estimated by an adaptive algorithm such asa normalized least mean square (NLMS) algorithm.

A path estimation ĥ(t) is then convolved with an available referencesignal, i.e. the speaker signal x(t), and the convolution is subtractedfrom a microphone signal d(t).

The adaptive filter (AF) tries to filter a speaker signal in the sameway that filtration is performed by a physical environment through whichspeaker oscillations propagate to a microphone 21. The result of theforegoing subtraction e(t) is used for filter adaptation. For anestimated echo path ĥ(t), in the absence of any additional noiseincluding a near-end speech, e(t) tends to zero.

A limitation in performance of the acoustic echo canceler (AEC) 210arises from the fact that a finite number of coefficients (taps) of anAF cannot fully estimate the real echo path h(t), which is theoreticallyinfinite or, at least, contains significant values at a very longdecaying tail. Another limitation is due to linearity of the finiteimpulse response adaptive filter (FIR AF), so that the finite impulseresponse adaptive filter (FIR AF) is unable to take nonlinear frequencycomponents of echo into account.

Therefore, the error signal e(t) usually contains some residual echoes,which may be suppressed (i.e. secondary echo-cancellation) by a residualecho suppressor (RES) 220 shown in FIG. 2.

Typically, the residual echo suppressor (RES) 220 is related with speechenhancement for noisy environments. The residual echo suppressor (RES)220 estimates a noise intensity λd in an input signal, and estimates aprobability of speech presence p, and, given these two values,constructs an appropriate gain providing optimal speech amplificationand noise suppression. These operations may be performed in a frequencydomain for each short frame of a sound signal.

Thus, the residual echo suppressor (RES) 220 estimates a residual echointensity λe in the error signal e(t) given the reference signal x(t)and the estimated echo path ĥ(t) from the acoustic echo canceler (AEC)210. The residual echo is considered as an independent noise, and theintensity λe of the residual echo is used in the same manner as theforegoing noise intensity λd.

Since the residual echo suppressor (RES) 220 works in the frequencydomain, each frame of the error signal e(n) is converted into thefrequency domain by a discrete Fourier transform. Then the resultingvalue E(l, k), where l is a number of a frame and k is a frequency binindex, is processed by an algorithm for each k, for example, by a Wienerfilter or an algorithm based on minimum mean square error (MMSE)estimation. The algorithm may be predetermined. For the error signal andother signals, a discrete variable n is used instead of a time variablet, because the processing is performed on discrete samples of digitalsignals.

The residual echo suppressor (RES) 220 may employ estimation proceduresfor the reverberation time T60(l, k) and initial power c(l, k) of theresidual echo, and thus may have a disadvantage by having thecomputational complexity of λe(l, k) estimation.

FIG. 3 is a block diagram showing an echo cancelation device thatcomplements operations of the residual echo suppressor of FIG. 2according to an exemplary embodiment.

In the exemplary embodiment shown in FIG. 3, in order to improve thecomputational complexity in the residual echo suppressor (RES) 220 ofFIG. 2, the acoustic echo path ĥ(n) may be first filtered by a number ofnarrow band-pass filters (FIG. 2), and algorithms are then applied toresulting sub-band echo paths ĥ(n, b), where b denotes the sub-bandnumber, as shown in FIG. 3.

Specifically, a number of points of an energy decay curve (EDC) for asub-band impulse response may be calculated. Each point is calculated bythe following Formula 1.

$\begin{matrix}{{C_{m} = {20{\log_{10}\left( {\sum\limits_{j = m}^{N - 1}\mspace{11mu} {{\hat{h}}^{2}\left( {j,b} \right)}} \right)}}},} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

for m=0, 1, . . . N−1,

where, N denotes a length of a corresponding sub-band echo path ĥ(n,b).

Then, linear approximation of a set of C_(m) points is performed, whichuses calculation of regression coefficients:

${\sum\limits_{j = m_{s}}^{m_{e}}\; C_{j}},{\sum\limits_{j = m_{s}}^{m_{e}}\; C_{j}^{2}},{\sum\limits_{j = m_{s}}^{m_{e}}\; {{jC}_{j}\mspace{14mu} {and}\mspace{14mu} {\sum\limits_{j = m_{s}}^{m_{e}}\; j}}},$

where m_(s) and m_(e) denote start-time and end-time respectively, insense of discrete time scale for ĥ(n,b) and its EDC. The foregoingoperations are performed for each sub-band b.

In the residual echo suppressor (RES) 320 of FIG. 3, estimation of theinitial power c(l, k) may be performed for each frequency index k inaccordance with the following Formula 2.

$\begin{matrix}{{c\left( {l,k} \right)} = {{A(k)}{{\sum\limits_{j = 0}^{N_{w} - 1}\; {{\hat{h}\left( {N - N_{w} + j} \right)}{\exp \left( {{- }\frac{2\pi \; k}{N_{DFT}}j} \right)}}}}^{2}}} & \left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack\end{matrix}$

Where, A(k) denotes a certain coefficient depending on T60(l, k), N_(w)is a certain number of ĥ(n) samples in its tail (N_(w)<N), i=+√{squareroot over (−1)}, N_(DFT) is a length of the used discrete Fouriertransform.

The foregoing residual echo suppressor (RES) 320 of FIG. 3 reduces thecomputational complexity more than the residual echo suppressor (RES)220 of FIG. 2. The computations of the residual echo suppressor (RES)320 may become complicated in real-time systems for the acoustic echocancellation.

FIG. 4 is a block diagram showing an echo cancelation device thatcomplements operations of the residual echo suppressors of FIG. 2 andFIG. 3 according to an exemplary embodiment, and FIG. 5 is a viewshowing a waveform of an impulse response signal according to anexemplary embodiment.

The exemplary embodiment shown in FIG. 4 provides an apparatus andmethod of full-band residual echo suppression after a sub-band acousticecho cancellation.

In a smart TV capable of supporting the voice recognition function, avoice command is input to a TV microphone simultaneously withsurrounding noise, where a main source of the surrounding noise is a TVspeaker playing a broadcast (i.e. a TV echo).

The voice recognition engine is susceptible to noise. Therefore, thesmart TV or the like electronic apparatus supporting the voicerecognition function seeks to remove the TV echo from an input signalreceived in the microphone as much as possible, while preserving voicecommands to be analyzed for recognition.

An acoustic echo canceler (AEC) 410 may work in a double-talk scenario.For example, a near-end speech s(n) and a far-end speech x(t), arenon-zero simultaneously. For example, the near-end speech s(n) may be acommand. This scenario uses non-trivial adaptive algorithms forprocessing the microphone signal in real time.

The complexity of AEC filtration algorithms increases as L2 or at leastL·log L, where L is a filter length.

For the adaptive filters, a long overlap between successive signalframes is only applicable to track rapid signal or environment changes,and it is therefore not so easy to gain from usage of frequency domainoperations, which provide the L·log L complexity. Also, it may bedifficult to reduce an adaptive filter length L in case of the AECwithout output result degradation.

For this reason, sub-band AEC algorithms are popular in real-timesystems.

Specifically, the acoustic echo canceler (AEC) 410 for the primaryecho-cancellation divides an input signal of an impulse response into aplurality of sub-bands to generate a plurality of sub-frames, andapplies adaptive filtering to each of the divided sub-bands (i.e.sub-frames) to estimate the acoustic echo path, thereby estimating anecho by convolution of the estimated acoustic echo path and the speakersignal. This convolution is performed with regard to each sub-band.

Thus, the echoes according to the sub-bands are summed into a full-bandecho, and the primary echo-cancellation is performed in such a mannerthat the full-band echo is removed from the input signal.

In this exemplary embodiment, the acoustic echo canceler (AEC) 410 mayinclude 16 sub-band AECs for generating 16 sub-frames. However, this isonly an example. The number of sub-bands is not particularly limited,and the number of sub-bands may be greater than or less than 16.

The acoustic echo canceler (AEC) 410 applies primary echo-cancellationto each of the sub-bands included in the domain to be analyzed, in whichthe domain to be analyzed may be a section excluding a final residualecho section corresponding to a decaying tail of the impulse responseand, for instance, may include a direct sound section and an initialreflected sound section.

The residual echo suppressor (RES) 420 for the secondaryecho-cancellation estimates a residual echo of an input signal, i.e., animpulse response, based on information about the acoustic echo pathobtained by the primary echo-cancellation with respect to each of theplural sub-bands (hereinafter, referred to as sub-frames), and performsthe secondary echo-cancellation for removing the estimated residual echofrom the input signal from which the echo is primarily removed. Theresidual echo suppressor (RES) 420 can estimate the residual echo in thefinal residual echo section corresponding to the decaying tail of theimpulse response based on the information about the acoustic echo pathestimated for each of the plural sub-bands of the domain (a) to beanalyzed.

Since the input signal of the microphone is actually given in the formof an infinite impulse response, the residual echo in the final residualecho section corresponding to the infinite decaying tail (not shown) isestimated based on information estimated by the acoustic echo canceler(AEC) 410 about the acoustic echo path of the domain to be analyzed,thereby reducing the computational complexity in estimating and removingthe residual echo in the impulse response of FIG. 5 to be describedlater.

As shown in FIG. 4, the acoustic echo canceler (AEC) 410 according to anexemplary embodiment includes a first sub-band generation module(Analysis) 411, a second sub-band generation module (Analysis) 412, afull-band synthesis module (Synthesis) 413, and an echo estimationmodule 414.

In the acoustic echo canceler (AEC) 410 of FIG. 4 as compared with thatof FIG. 2 or 3, an adaptive filter in each sub-band can be quite short,at the same time preserving a value of a total length, which is a sum ofsub-band lengths. However, even after the operation of the acoustic echocanceler (AEC) 410, a residual echo still remains in a microphone signaland therefore the operation of the residual echo suppressor (RES) 420 isused like those of FIGS. 2 and 3. Accordingly, the RES system is alsosuitable for echo suppression in the smart TV.

According to the exemplary embodiment shown in FIG. 4, the residual echosuppressor (RES) 420 is provided for the sub-band acoustic echo canceler(AEC) 410 and removes the residual echo. For the estimation of values bythe residual echo suppressor (RES) 420, sub-band impulse responses areused. Thus, an efficient estimation algorithm may be suggested for thesevalues.

In FIG. 4, the sub-band acoustic echo canceler (AEC) 410 receives themicrophone signal (i.e. the input signal) d(n) as well as the speakersignal (i.e. the reference signal x(n)). The input signal d(n) isdivided into sub-bands by the first sub-band generation module(Analysis) 411, and the reference signal x(n) is divided by intosub-bands by the second sub-band generation module (Analysis) 412,thereby generating a plurality of sub-bands (for example, M sub-bands).In other words, the input signal d(n) and the reference signal x(n) areanalyzed into sub-frames. The first and second sub-band generationmodules (Analysis) 411 and 412 may include M band-pass filters anddown-sampling units.

The echo estimation module 414 estimates the acoustic echo path byperforming adaptive filtration in each sub-band based on the outputsfrom the first and second sub-band generation modules (Analysis) 411 and412. Further, the echo estimation module 414 estimates the echo byconvolving the estimated sub-band acoustic echo path with the speakersignal x(t) (i.e., the reference signal x(n)). Here, the adaptivealgorithm, for example, the FIR AF described with reference to FIG. 2,may be used for the adaptive filtration.

The filtered signals of the sub-bands, that is, the estimated echoes arecollected into the full-band signal by the full-band synthesis module(Synthesis) 412. The full-band synthesis module (Synthesis) 412 mayinclude M up-sampling units and band-pass filters for preventing animaging effect of up-sampling.

Then, an echo h(n)*x(n) estimated with respect to the full-band isprimarily removed from the input signal, i.e. from the impulse responsed(n).

During the operations of the AEC in the foregoing sub-bands, a set ofimpulse responses ĥ_(b)(n) is estimated, where b=0, 1, . . . , M−1, andM is the number of sub-bands, i.e. sub-frames.

In this exemplary embodiment, as shown in FIG. 4 the residual echosuppressor 420 receives the impulse response ĥ_(b) (n) of each sub-bandestimated by the acoustic echo canceler 410, and uses the receivedimpulse response to estimate the residual echo, thereby performing thesecondary echo-cancellation to remove the estimated residual echo.

According to an exemplary embodiment, the residual echo suppressor 420of FIG. 4 uses an exponential decay coefficient ρ(l, k) of the impulseresponse shown in FIG. 5. In FIG. 5, the horizontal axis (x axis)corresponds to the number of samples, i.e. time (T), the vertical axis(y axis) corresponds to energy (E), and a coefficient ρ(l, k) isconnected with a reverberation time T60(l, k) as ρ(l, k)=3·ln 10/T60(l,k).

In this exemplary embodiment, the echo cancelation device 400 operatesbased on a ρ_(b)(l) calculation algorithm (to be described later), andhas a connection scheme between the sub-band acoustic echo canceler(AEC) 410 and the full-band residual echo suppressor (RES) 420 in orderto get the sub-band decaying coefficient ρ_(b)(l) from ĥ_(b) (n)estimated corresponding to each sub-band as shown in FIG. 4.

FIG. 6 is a flowchart showing operations based on an algorithm forestimating a sub-band impulse response decaying coefficient ρ_(b)(l) inthe residual echo suppressor (RES) of FIG. 4.

The algorithm according to this exemplary embodiment shown in FIG. 6provides a result in one cycle over samples of ĥ_(b) (n), and may bemore convenient in practice than that of FIG. 3.

Referring to FIGS. 5 and 6, the algorithm of FIG. 6 is to find a maximalabsolute value H_(max) 51 of the impulse response ĥ_(b)(n) in abeginning portion of the impulse response, and then to find thecorresponding local maximum H_(min) at a decaying tail of an absolutevalue of the impulse response |ĥ_(b)(n)|. In other words, this algorithmfinds the highest local maximum at the end of the impulse response (IR).

These two values H_(max) and H_(min) satisfy the following Formula 3using an exponential function.

$\begin{matrix}{H_{\min} = {H_{\max}\exp \left\{ {{- \rho_{b}}\frac{i_{{mi}n} - i_{\max}}{f_{sb}}} \right\}}} & \left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack\end{matrix}$

where, i_(min) and imax denote indexes of H_(min) and H_(max)respectively.

As shown in FIG. 5, the global maximum H_(max) of an impulse responsemodule corresponding to the curve |ĥ_(b)(n)| is found. In addition, thefollowing two conditions have to be satisfied to obtain H_(min).

1) It has to be a local maximum.

2) In the vicinity there can be other local maximums, which are lessthan a desired one.

These conditions are based on an observation that the impulse responsesgenerally have decaying repetitions of some initial patterns.

The foregoing algorithm for estimating the sub-band impulse response(IR) decaying coefficient according to the exemplary embodiment shown inFIG. 6 operates as follows.

Operation S0. Initialization is performed. For example, initializeL_(search)=L_(b)/D, where D is an integer value. Initialize:H_(max)=Max₁=Max₂=−1; i₁=i₂=i_(max)=0; i=0; m=0.

Operation S10. If (i==L_(b)−1), then go to Operation S80.

Operation S20. If (|ĥ_(b)(i)|>H_(max)), then: H_(max)=|ĥ_(b)(i)|;i_(max)=i₁=i; m=0; Max₁=Max₂=−1; go to Operation S70.

Operation S30. If (|ĥ_(b)(i)|≦|ĥ_(b)(i−1)| or |ĥ_(b)(i)|<|ĥ_(b)(i+1)|),then: m=m+1; go to Operation S70.

Operation S40. If (|ĥ_(b)(i)|>Max₁), then: Max₁=|ĥ_(b)(i)|; i₁=i;Max₂=−1; m=0; go to Operation S70.

Operation S50. If (|ĥ_(b)(i)|>Max₂), then: Max₂=|ĥ_(b)(i)|; i₂=i; m=m+1.

Operation S60. If (m≧L_(search)), then: m=0; Max₁=Max₂; i₁=i₂; Max₂=−1.

Operation S70. i=i+1. Go to Operation S10.

Operation S80. Return: ρ_(b)=f_(sb)·ln(H_(max)/Max₁)/(i₁−i_(max)), wheref_(sb) is a sampling frequency in a band b, which, due to down-sampling,is M times less than the input signal sampling frequency.

As shown in FIG. 6 and the foregoing algorithm, in the Operation S0,initialization is performed with L_(search)=L_(b) D, where D is aninteger value. Further, initialization is performed withH_(max)=Max₁=Max₂=−1; i₁=i₂=i_(max)=0; i=0; m=0.

Here, L_(search) is to divide ĥ_(b) (n) estimated with regard to eachsub-band into a number of search windows. The number of search windowsmay be predetermined. For example, at L_(b)=256 or L_(b)=64, goodresults are obtained with D=8. That is, L_(b) may correspond to a domainto be analyzed, as a length of the impulse response ĥ_(b) (n) of eachsub-band.

On the assumption that H_(max) and its index i_(max) are invariables,the maximal absolute value of ĥ_(b) (n) is found. The indexes i₁ and i₂corresponding to the variables Max₁ and Max₂ are used for searching thelocal maximums of ĥ_(b) (n) after H_(max). The initial value for thevariables H_(max), Max₁ and Max₂ is “−1” since the initial value shouldbe smaller than any positive or zero number. The variable i is used foran index sample of the impulse response. The variable m is used fortracking a certain search window.

Next, in the Operation S10, i and L_(b) are compared. If i<L_(b), theOperation S20 starts. If i==L_(b)−1, the Operation S80 directly starts.

That is, if the end of the impulse response ĥ_(b) (n) as shown in FIG. 5is reached once in the Operation S10, the result may be calculated byjumping to the Operation S80. Otherwise, the Operation S20 starts. Forexample, if the Operation S10 starts directly after the initializationis performed with i=0 in the operation S0, the Operation S20 starts inaccordance with determination results in the Operation S10.

In the Operation S20, |ĥ_(b)(i)| and H_(max) are compared. If|ĥ_(b)(i)|>H_(max), the Operation S21 starts to perform updating withH_(max)=|ĥ_(b)(i)| and resetting with i_(max)=i₁=i; m=0; Max₁=Max₂=−1,and then proceeds to the Operation S70. In the Operation S20, if|ĥ_(b)(i)|≦H_(max), the Operation S30 starts.

The Operation S20 is to search for H_(max). If this operation isperformed once, the Operation S20 is skipped in accordance with checkedstates, and the present algorithm performs the following steps to searchfor some local maximum corresponding to H_(max) for each |ĥ_(b)(i)|.That is, the Operation S30 directly follows the Operation S20 afterdetermining H_(max) in the beginning of the impulse response shown inFIG. 5.

In the Operation S30, it is determined whether there is the localmaximum of |ĥ_(b)(i)|. Specifically, comparison is performed between|ĥ_(b)(i)| and |ĥ_(b)(i−1)| and between |ĥ_(b)(i)| and |ĥ_(b)(i+1)|. Asa result of the comparison, if (|ĥ_(b)(i)|≦|ĥ_(b)(i−1)| or|ĥ_(b)(i)|<|ĥ_(b)(i+1)|, the Operation S31 starts to get m=m+1 (i.e.,m++), and proceeds to the Operation S70. That is, if |ĥ_(b)(i)| is notthe local maximum, the operations for searching the local maximum areperformed with regard to the next |ĥ_(b)(i)| by increasing the index mby 1 in the corresponding search window at the Operation S31 andincreasing the index i by 1 at the Operation S70.

If the local maximum is found as the comparison result in the OperationS30, e.g., if |ĥ_(b)(i)|>|ĥ_(b)(i−1)| or |ĥ_(b)(i)|≧|ĥ_(b)(i+1)|, theOperation S40 may start.

In the Operation S40, |ĥ_(b)(i)| and Max₁ are compared. if|ĥ_(b)(i)|>Max₁, the Operation S41 starts to perform updating withMax₁=|ĥ_(b)(i)| and resetting with i₁=i; Max₂=−1; m=0 and proceeds tothe Operation S70.

That is, if it is determined in the previous Operation S30 that|ĥ_(b)(i)| is the local maximum, it is checked whether or not |ĥ_(b)(i)|is higher than the previous value of Max₁. As a checking result, if|ĥ_(b)(i)| is higher than Max₁, Max₁ and its index i are updated, andMax₂ and m are reset.

In practice, Max₁ is to store a value of a global maximum found in thelocal search interval (window), whereas Max₂ is to search thisglobal-on-interval local maximum.

The reason is as follows. If Max₁ is updated, Max₂ is not needed in thecurrent search window and thus the next Operation S50 is skipped. SinceMax₂ is also unnecessary in the search window of the Operation S50, m isreset and the Operation S60 is skipped. Resetting of m in this operationmeans that a new search window starts directly after a new value ofMax₁.

In other words, this algorithm stores H_(max) as Max₁ when H_(max) isdetermined in the corresponding search window, and continues to performthe operations of the Operations S10 to S40 corresponding to therespective conditions with respect to the next window without performingthe operations following the Operation S50 for determining the localmaximum H_(min) in the corresponding search window.

On the other hand, if |ĥ_(b)(i)| is equal to or lower than Max₁ in theOperation S40, the Operation S50 starts to search the local maximum inthe corresponding search window.

In the Operation S50, |ĥ_(b)(i)| and Max₂ are compared. If|ĥ_(b)(i)|>Max₂, the Operation S51 starts to perform updating withMax₂=|ĥ_(b)(i)| and resetting with i₂=i; m=m+1(m++) and proceeds to theOperation S60.

The move from the Operation S50 to the Operation S51 means that|ĥ_(b)(i)| corresponds to the local maximum and is lower than Max₁. If|ĥ_(b)(i)| is higher than the previous value of Max₂, Max₂ and its indexi₂ have to be properly updated. This is a procedure of searching theglobal maximum within the bounded search window.

On the other hand, if |ĥ_(b)(i)|≦Max₂, the Operation S50 proceeds to theOperation S60. In the Operation S60, m and L_(search) are compared. As acomparison result, if m≧L_(search), the Operation S61 starts to havem=0, Max₁=Max₂, i₁=i₂, Max₂=−1.

That is, in the next search window, a value of Max₂ is stored in Max₁ sothat Max₂ can be freely variable for the following search.

The Operation S70 increases i by i=i+1(i++) and proceeds to theOperation S10, which means an increase in an index of an impulseresponse array.

If the increase in the Operation S70 satisfies i==L_(b)−1 in theOperation S10, the Operation S80 starts.

In the Operation S80, ρ_(b) is returned by the following Formula 4.

ρ_(b) =f _(sb)·ln(H _(max)/Max₁)/(i ₁ −i _(max))  [Formula 4]

where, f_(sb) denotes a sampling frequency in a band b, which is M timesless than the input signal sampling frequency due to down-sampling.

In the Operation S80, Max₁ and i₁ are used as H_(min) and i_(min),respectively, in the formula written prior to the steps of thisalgorithm.

As shown in FIG. 6, the Operation S70 starts to increase i and proceedsto the Operation S10 after updating/resetting Max₁, Max₂ and m in theOperation S21, S31, S41 and S61.

The foregoing algorithm is performed by the RES 420 with regard to eachsub-band based on information received from the AEC 140. That is, thealgorithm determines the maximum coefficient H_(max) and the localmaximum H_(min) by updating Max₁ and Max₂ while increasing i and m inresponse to conditions of the respective operations with respect to thesearch windows L_(search) divided from |ĥ_(b)(i)| corresponding to asub-band, and determines a decaying coefficient ρ_(b) of the impulseresponse with regard to each sub-band by applying the determined valuesto the Formula 4. The sub-band may be predetermined.

The decaying coefficient ρ_(b) is a slope of a straight linecorresponding to the logarithm of the square of the absolute magnitudeof the impulse responses shown in FIG. 5.

In the foregoing algorithm shown in FIG. 6, a computational complexity(i.e., the number of mathematical operations) is less than that in thecalculation approach for T60(l, k) provided in the exemplary embodimentof FIG. 3.

If ρ_(b) is calculated as described above, its value is assigned to ρ(l,k) for the current signal frame l and each frequency domain index kcorresponding to the bth sub-band of the processed IR ĥ_(b)(n).

Initial power c(l, k) of the residual echo is estimated by the followingFormula 5 using a sub-band IR ĥ_(b)(n).

$\begin{matrix}{c_{b} = {\exp \left\{ {- \frac{2\rho_{b}N_{w}}{f_{sb}}} \right\} {\sum\limits_{j = 0}^{N_{w} - 1}\; {{{\hat{h}}_{b}^{2}\left( {L_{b} - N_{w} + j} \right)}.}}}} & \left\lbrack {{Formula}\mspace{14mu} 5} \right\rbrack\end{matrix}$

where, Cb denotes the initial power in the corresponding sub-band, ρ_(b)denotes the decaying coefficient of the sub-band impulse response, L_(b)denotes the length of the sub-band impulse response, and N_(w) denotesthe length of the decaying tail of the sub-band impulse responseĥ_(b)(n).

That is, Cb is a value obtained by covering the tail of each sub-bandimpulse response with an energy decay curve, and corresponds to aninitial power estimation value at the tail of the impulse response underan actual environment (not shown) in the impulse response signal shownin FIG. 5 due to a finite number of taps.

In the case of using the Formula 5, calculation is performed once persub-band in order to estimate the initial power, and is thus less incomplexity than that using the Formula 2 in the exemplary embodiment ofFIG. 3. Further, the values of the initial power of the residual echoare simply retrieved as c(l, k)=cb/Nf.

Here, Nf denotes a number of indexes k corresponding to a bth sub-band.The index k refers to a frequency index of Fourier transform. If Fouriertransform is performed with regard to N sampling signals, it is possibleto obtain N frequency components and the indexes from 1 to N correspondto a frequency band obtained by dividing the Nyquist frequency from zeroby N.

According to an exemplary embodiment, the residual echo suppressor (RES)420 calculates the initial power Cb of each sub-band through theforegoing Formula 5, and estimates the residual echo corresponding tothe final residual echo section corresponding to the tail (not shown) ofthe impulse response of FIG. 5 through the calculated initial power Cband decaying coefficient ρ_(b).

Using the estimated residual echo, the secondary echo-cancellation isperformed to remove the residual echo from the full-band input signal.At this time, the residual echo suppressor 420 generates the full-bandresidual echo signal based on the residual echo estimated for eachsub-band, and receives the primary echo-cancelled signal from theacoustic echo canceler 410. Then, the secondary echo-cancellation isperformed in such a manner that the full-band residual echo signal isremoved from the received signal that has experienced the primaryecho-cancellation.

The present inventive concept may be applied to a stereo ormulti-channel AEC system that includes two near-end speakers. As thenumber of speakers becomes greater, the computational complexity growsalmost linearly.

In de-reverberation systems after the AEC, where estimation of areverberation time T60 is performed, the foregoing algorithm can be usedfor average T60 estimation in the full-band, given an impulse responseof an environment of interest.

The foregoing RES according to an exemplary embodiment reduces REScomplexity to thereby improve performance of a multi-reference AECsystem for a smart TV. AEC algorithms, optimized in sense of performanceand computational complexity, can be implemented by less expensive audioprocessing chips, thus saving money. At the same time, the algorithmsallow faster reaction on a customer's problem.

As described above, according to an exemplary embodiment, an echo signalis easily and effectively removed from an input signal received in amicrophone by primary echo-cancellation (i.e. the acoustic echocancellation of the acoustic echo canceller (AEC)) and secondaryecho-cancellation (i.e. the residual echo suppression of the residualecho suppressor (REC)), thereby improving performance of acoustic echosuppression in a frequency domain.

In addition, the secondary echo-cancellation employs informationobtained in the primary echo-cancellation to decrease the computationalcomplexity and memory usage and improve processing speed, therebyenhancing a user's convenience.

The foregoing exemplary embodiments may be implemented by a programstored on a computer-readable recording medium. The computer-readablerecording medium includes a transmission medium and a storage medium forstoring data readable by a computer system. The transmission medium maybe a wired/wireless network where computer systems are connected to oneanother.

The foregoing exemplary embodiments may be implemented by hardware or bya combination of hardware and software. The hardware including theAEC/RES includes a nonvolatile memory where software, i.e. a computerprogram is stored; a random access memory (RAM) to which the computerprogram stored in the nonvolatile memory is loaded; and at least oneprocessor for executing the computer program loaded to the RAM. Theprocessor may be a microprocessor, a microcontroller, or the like. Thenonvolatile memory may include a hard disk drive, a flash memory, a readonly memory (ROM), compact disc (CD)-ROMs, magnetic tapes, a floppydisk, an optical storage, a data transmission device using Internet,etc., but not limited thereto. The nonvolatile memory is an example ofthe computer-readable recording medium in which a program readable by acomputer is recorded.

The computer program is code that can be read and executed by theprocessor, and includes codes for enabling the processor provided as theAEC or RES in the apparatus to perform operations.

The computer program may be involved in software including an operatingsystem or applications provided in the display apparatus and/or softwareinterfacing with the external apparatus.

Although a few exemplary embodiments have been shown and described, itwill be appreciated by those skilled in the art that changes may be madein these exemplary embodiments without departing from the principles andspirit of the inventive concept, the scope of which is defined in theappended claims and their equivalents.

What is claimed is:
 1. A display apparatus comprising: a displayconfigured to display an image; a video receiver configured to receivean input signal of an impulse response; and at least one processorconfigured to divide the input signal into a plurality of sub-bands,apply primary echo-cancellation to the input signal by using a pluralityof sub-band signals corresponding to the plurality of sub-bands,estimate a residual echo based on acoustic echo path informationobtained in the primary echo-cancellation with regard to each of theplurality of sub-bands, and perform secondary echo-cancellation toremove the estimated residual echo from the primary-echo-canceled inputsignal.
 2. The display apparatus according to claim 1, wherein the atleast one processor comprises: an acoustic echo canceler configured toperform the primary echo-cancellation to obtain the acoustic echo pathinformation; and a residual echo suppressor configured to perform thesecondary echo-cancellation based on the acoustic echo path informationobtained by the acoustic echo canceler.
 3. The display apparatusaccording to claim 2, wherein the acoustic echo canceller is configuredto estimate the acoustic echo path information for each of thesub-bands, and the residual echo suppressor is configured to determine adecaying coefficient of an impulse response corresponding to each of theplurality of sub-bands, which has experienced the primaryecho-cancellation, based on the estimated acoustic echo path informationfor the sub-bands.
 4. The display apparatus according to claim 3,wherein the residual echo suppressor is configured determine a maximumof the impulse response and a local maximum at a decaying tail of theimpulse response for each sub-band, and determine the decayingcoefficient based on the determined maximum and local maximum.
 5. Thedisplay apparatus according to claim 4, wherein the residual echosuppressor is configured to determine the maximum and local maximum foreach sub-band by dividing the impulse response corresponding to eachsub-band into a plurality of search windows and updating the maximum orthe local maximum while increasing an index with regard to each dividedsearch window.
 6. The display apparatus according to claim 5, whereinthe residual echo suppressor is configured to skip a procedure fordetermining the local maximum in a corresponding search window if themaximum is determined in a beginning of the impulse response, and updatethe local maximum in a next search window.
 7. The display apparatusaccording to claim 5, wherein the decaying coefficient is determined by:ρ_(b) =f _(sb)·ln(H _(max)/Max₁)/(i ₁ −i _(max)), where ρ_(b) denotesthe decaying coefficient, H_(max) denotes a maximum of the impulseresponse, i_(max) denotes an index of H_(max), Max₁ denotes anultimately updated local maximum, i₁ denotes an index of the localmaximum, and f_(sb) denotes a sampling frequency due to down-sampling ofthe impulse response.
 8. The display apparatus according to claim 3,wherein the residual echo suppressor is configured to estimate initialpower in a final residual echo section of the impulse responsecorresponding to each of the plurality of sub-bands, which hasexperienced the primary echo-cancellation, based on the decayingcoefficient.
 9. The display apparatus according to claim 8, wherein theinitial power in each of the plurality of sub-bands is determined by:$c_{b} = {\exp \left\{ {- \frac{2\rho_{b}N_{w}}{f_{sb}}} \right\} {\sum\limits_{j = 0}^{N_{w} - 1}\; {{{\hat{h}}_{b}^{2}\left( {L_{b} - N_{w} + j} \right)}.}}}$where Cb denotes initial power, ρ_(b) denotes a decaying coefficient ofa sub-band impulse response, L_(b) denotes a length of the sub-bandimpulse response, N_(w) denotes a length of a decaying tail of thesub-band impulse response, f_(sb) denotes a sampling frequency due to adown-sampling of an impulse response, and ĥ_(b)(n) denotes the sub-bandimpulse response.
 10. The display apparatus according to claim 2,wherein the residual echo suppressor is configured to estimate anacoustic echo path by applying adaptive filtration to each of theplurality of sub-bands, estimate an echo by convolving the estimatedacoustic echo path for the sub-bands with a speaker signal, and remove afull-band echo signal synthesized with an echo estimated for thesub-bands from the input signal.
 11. An echo cancellation method of adisplay apparatus, the method comprising: receiving an input signal ofan impulse response; dividing the input signal into a plurality ofsub-bands, and applying primary echo-cancellation to the input signal byusing a plurality of sub-band signals corresponding to the plurality ofsub-bands; and estimating a residual echo based on acoustic echo pathinformation obtained in the primary echo-cancellation with regard toeach of the plurality of sub-bands, and performing secondaryecho-cancellation to remove the estimated residual echo from theprimary-echo-canceled input signal.
 12. The echo cancellation methodaccording to claim 11, wherein the applying the primaryecho-cancellation to the input signal comprises estimating acoustic echopath information for each of the plurality of sub-bands, and theperforming the secondary echo-cancellation comprises determining adecaying coefficient of an impulse response corresponding to each of theplurality of sub-bands, which has experienced the primaryecho-cancellation, based on the estimated acoustic echo path informationfor the sub-bands.
 13. The echo cancellation method according to claim12, wherein the performing the secondary echo-cancellation comprisesdetermining a maximum of the impulse response and a local maximum at adecaying tail of the impulse response for each sub-band, and determiningthe decaying coefficient based on the determined maximum and localmaximum.
 14. The echo cancellation method according to claim 13, whereinthe maximum and local maximum is determined for each sub-band bydividing the impulse response corresponding to each sub-band into aplurality of search windows and updating the maximum or the localmaximum while increasing an index with regard to each divided searchwindow.
 15. The echo cancellation method according to claim 14, whereinthe performing the secondary echo-cancellation comprises: skipping aprocedure for determining the local maximum in a corresponding searchwindow if the maximum is determined in a beginning of the impulseresponse; and updating the local maximum in a next search window. 16.The echo cancellation method according to claim 14, wherein the decayingcoefficient is determined by:ρ_(b) =f _(sb)·ln(H _(max)/Max₁)/(i ₁ −i _(max)), where ρ_(b) denotesthe decaying coefficient, H_(max) denotes a maximum of the impulseresponse, i_(max) denotes an index of H_(max), Max₁ denotes anultimately updated local maximum, i₁ denotes an index of the localmaximum, and f_(sb) denotes a sampling frequency due to down-sampling ofthe impulse response.
 17. The echo cancellation method according toclaim 12, wherein the performing the secondary echo-cancellationcomprises estimating initial power in a final residual echo section ofthe impulse response corresponding to each of the plurality ofsub-bands, which has experienced the primary echo-cancellation, based onthe decaying coefficient.
 18. The echo cancellation method according toclaim 17, wherein the initial power in each of the plurality ofsub-bands is determined by:$c_{b} = {\exp \left\{ {- \frac{2\rho_{b}N_{w}}{f_{sb}}} \right\} {\sum\limits_{j = 0}^{N_{w} - 1}\; {{{\hat{h}}_{b}^{2}\left( {L_{b} - N_{w} + j} \right)}.}}}$where Cb denotes initial power, ρ_(b) denotes a decaying coefficient ofa sub-band impulse response, L_(b) denotes a length of the sub-bandimpulse response, N_(w) denotes a length of a decaying tail of thesub-band impulse response, f_(sb) denotes a sampling frequency due to adown-sampling of the impulse response, and ĥ_(b)(n) denotes the sub-bandimpulse response.
 19. The echo cancellation method according to claim11, wherein the applying the primary echo-cancellation comprises:estimating an acoustic echo path by applying adaptive filtration to eachof the plurality of sub-bands; estimating an echo by convolving theestimated acoustic echo path for the sub-bands with a speaker signal;and removing a full-band echo signal synthesized with an echo estimatedfor the sub-bands from the input signal.
 20. An echo cancellation devicecomprising: at least one processor configured to perform primaryecho-cancellation on an input signal by dividing the input signal into aplurality of side-bands, removing a primary echo from each of theplurality of side-bands to produce a primary-echo-cancelled signal foreach side-band, and estimating a residual echo for each of theprimary-echo-cancelled signals; and at least one processor configured toperform residual echo-cancellation by generating a full-band residualecho signal from the estimated residual echoes for each side-band, andremove the full-band residual echo signal from a signal on which theprimary echo-cancellation has been performed.